Decreasing water supplies throughout much of the industrialized world necessitate new methods and systems for utilizing water including contaminants and impurities. Additionally, certain industries have a need for safer, more energy efficient methods and systems for removing water from a target material or solute. Conventional methods and systems of aqueous liquid treatment include thermal flash evaporation and membrane filtration. The most popular membrane filtration method is reverse osmosis, in which water is separated from solutes (e.g., contaminants) in an aqueous feed liquid by application of a pressure overcoming the osmotic pressure of the aqueous feed liquid. For water desalination processes, the pressure to overcome the osmotic pressure of the contaminated water source can be substantial, such as greater than 50 atm, significant investment in equipment and substantial ongoing energy costs. Additionally, the application of pressure in reverse osmosis processes often exacerbates membrane fouling by inorganic and organic molecules.
Forward osmosis circumvents several of the deficiencies of reverse osmosis by using osmotic pressure gradients across a semi-permeable membrane to diffuse water from the aqueous feed liquid into a draw solution. The draw solution includes a draw solute that enables the draw solution to have a greater osmotic pressure than the aqueous feed liquid. Conventional draw solutes include sodium chloride, potassium chloride, magnesium chloride, calcium chloride, sulfur dioxide, magnesium sulfate, aluminum sulfate, sugars (e.g., glucose, fructose, sucrose), potassium nitrate, ammonium carbonate, and ammonium carbamate.
Unfortunately, the molecular weight and size of conventional draw solutes frequently results in back diffusion of the draw solute through the semi-permeable membrane, requiring added expense to replace the draw solute that is lost. The molecular weight and size of conventional draw solutes also typically requires costly and energy intensive processes, such as reverse osmosis, to remove the conventional draw solutes from the water they draw. Further, conventional draw solutes are generally non-ionic, monovalent, or divalent, meaning that the osmotic pressure they impart can be limited, as can be the flux of water through the semi-permeable membrane during forward osmosis.
It would, therefore, be desirable to have a relatively larger draw solute that does not back-diffuse through the semi-permeable membrane, and that may be removed from the water it draws by relatively less expensive separation processes. It would also be desirable for the draw solute to have relatively increased durability, and increased osmotic pressure as compared to conventional draw solutes. The use of such a draw solute would reduce the energy demands and operational costs associated with current water treatment technologies, including a broad array of operations such as bulk water purification, produced water (e.g., waters brought to the surface during oil and gas drilling) utilization, solution mining (e.g., mining of underground, water-soluble minerals), carbon dioxide scrubbing, and acid gas scrubbing.